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The genetic analysis of the heterochromatin of chromosome 3 of Drosophila melanogaster Marchant, Gary Elvin


The heterochromatin of the third chromosome is the largest uncharacterized region of the Drosophila we lanogaster genome, and the last major block of D. we lanogaster heterochromatin to be thoroughly analyzed. In the present study, this region was genetically dissected by generating and analyzing a series of attached, detached and reattached third chromosomes. Separate detachment experiments were conducted for all twelve possible combinations of four newly synthesized sister-strand compound-3L's and three newly synthesized sister-strand compound-3R' s. A total of 443 recessive lethal detachment products carrying putative heterochromatic deficiencies were tested for complementation in a several-stage complementation analysis. The results revealed the presence of seven separable vital regions in the heterochromatin of chromosome three. Attempts to reattach deficiency-carrying detachment products established that six of these vital regions are on the left arm, and only one is on the right arm. An analysis of the types and frequencies of detachment product deficiencies generated in each detachment experiment permitted the genetic characterization of the progenitor compounds. It was also possible to determine the proximal-distal orientation of the genes on each arm, and to identify possible breakpoints for each lethal detachment product produced. Seventy-five EMS-induced lethal alleles of detachment product deficiencies were also recovered and tested for complementation. Four additional genes in third chromosome heterochromatin were revealed, three on the left arm and one on the right arm. At least three of the EMS-induced lethals were small deficiencies. The inter-alleiic complementation observed between some EMS-induced lethals, as well as the recovery of a temperature sensitive mutation of a heterochromatic gene, provided further evidence that there are single-copy, transcribed vital genes in third chromosome heterochromatin. Finally, a cytological analysis of three of the detachment product deficiencies by L. Sandler and S. Pimpinelli (personal communication) provided evidence that at least some of the genes uncovered in this study are located in the most distal segments of third chromosome heterochromatin. The results suggest that the eleven vital genes discovered in the heterochromatin of the third chromosome are not randomly distributed between, nor within, the heterochromatic blocks of the left and right arms. This study provides further evidence that Drosophila heterochromatin is genetically heterogeneous, with vital genes being present in some heterochromatic segments but not others.

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